Return air

[DanW13]

How is the amount of return air calculated ? If a home has 12 supply lines all 6" for example what are the variables used to calculate what the total amount of return air that would be needed? Is it as simple as total supply air being push out equals the amount of return air ? Or does a system need more than the other weather it's more supply air vs. return air or more return air vs. supply air ?

[Ed Janowiak]

How is the amount of return air calculated ? If a home has 12 supply lines all 6" for example what are the variables used to calculate what the total amount of return air that would be needed? Is it as simple as total supply air being push out equals the amount of return air ? Or does a system need more than the other weather it's more supply air vs. return air or more return air vs. supply air ?

It's determined by volume (CFM) and speed (FPM).

The systems Total Effective Length is calculated, and that is divided into the the Available Static Pressure X 100. This will be your Friction Rate.

Once the Friction Rate has been calculated a duct slide rule is used to pick the correct size duct(s) that will move the required CFM at a velocity that will not cause noise complaints.

Recommended maximum velocities are slightly lower on the return side, so it is often said the return ducts have to be bigger. That is sometimes true and some times not, the duct slide rule when set properly will give you the answer.

This is a short answer to a complex question. Correct duct sizing is not an easy task, there's no wonder there are so many bad duct systems installed everyday.